1. Field of the Invention
The present invention relates to an electrostatographic encapsulated toner employable for producing a visible image from a latent image in a recording method utilizing an electrostatography.
2. Description of Prior Arts
As the process for fixing a toner image in a recording method such as electrostatography, there have been known three fixing processes, that is, a heat fixing process, a solvent fixing process and a pressure fixing process. Recently, the heat fixing process and the pressure fixing process, both using no solvent, are widely used from the viewpoint of the prevention of environmental pollution.
In the heat fixing process, a toner comprising a colorant bound with a binder has been conventionally employed. The same kind of toner is also employed in the pressure fixing process, but utilization of an encapsulated toner is recently proposed in the pressure fixing process.
The encapsulated toner is a toner in the form of micro-capsule prepared by enclosing a core material comprising a colorant such as carbon black and a binder with a resin shell which is rupturable by the application of pressure.
The conventional encapsulated toner is not necessarily satisfactory in various properties that are essentially required for a toner employable in the electrostatography.
In more detail, a toner employable as a developing agent in electrostatography is required to have various excellent properties such as high powder flowability, high developing efficiency, and no smearing of the surface of a photosensitive medium for producing a latent image. Further, in the case of a two-component developing process, it is necessary that the toner does not smear the surface of the employed carrier. In the pressure fixing process, high fixability, little occurrence of offsetting phenomenon on a pressure roller used in the process (namely, toner adheres to the surface of a pressure roller to stain the roller), etc. are also required for the toner.
Accordingly, the toner employed in the pressure fixing process should be satisfactory in all properties such as powder flowability, fixability to a supporting medium (e.g., paper), preservation stability of the fixed image, anti-offsetting property, and electrostatic chargeability and/or conductivity required depending upon a developing process. However, the conventional toners are not well satisfactory in the above-mentioned characteristics.
For instance, Japanese Patent Provisional Publication No. 54(1979)-76233 describes an encapsulated toner having colloidal silica, aluminum silicate or calcium silicate on the surface of the shell for improving the flowablity of the encapsulated toner. This toner is favorable in improving the flowablity, but no improvement is given with respect to the fixability.
The improvement of the fixability of the encapsulated toner is disclosed in Japanese Patent Provisional Publication No. 60(1985)-184259 in which a specific silicone is incorporated into the toner. The improvement of the fixability is based on the conception that the fixed toner image is arranged to provide a well sliding surface whereby keeping the toner image from dropping off caused by contact with other material. The silicone is introduced into the toner to give improved sliding function.
However, according to study of the present inventor, the incorporated silicone is apt to ooze out of the surface of the shell to produce agglomerates of toner particles. It has been further discovered that the silicone being dissolved in the binder system of the core material as disclosed in the above-mentioned publication does not give well improved sliding function.